The present invention relates generally to data transmission in mobile communication systems and more specifically to methods for re-synchronizing an uplink between a user agent and an access device.
As used herein, the terms “user agent” and “UA” can refer to wireless devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices that have telecommunications capabilities. In some embodiments, a UA may refer to a mobile wireless device. The term “UA” may also refer to devices that have similar capabilities but that are not transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station or access device transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an E-UTRAN (evolved universal terrestrial radio access network) node B (eNB), a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). As used herein, the term “access device” will refer to any component, such as a traditional base station or an LTE eNB (Evolved Node B), that can provide a UA with access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, the access device provides radio accesses to one or more UAs. The access device comprises a packet scheduler for allocating uplink and downlink data transmission resources among all the UAs communicating to the access device. The functions of the scheduler include, among others, dividing the available air interface capacity between the UAs, deciding the resources (e.g. sub-carrier frequencies and timing) to be used for each UA's packet data transmission, and monitoring packet allocation and system load. The scheduler allocates physical layer resources for downlink shared channel (PDSCH) and uplink shared channel (PUSCH) data transmissions, and sends scheduling information to the UAs through a scheduling channel. The UAs refer to the scheduling information for the timing, frequency, data block size, modulation and coding of uplink and downlink transmissions.
There are several ways to start an unscheduled communication between an access device and a UA which has already established a connection to the access device. Here two ways to start a communication are described including a first way initiated by a UA and a second way initiated by an access device. One of skill in the art should recognize that after UA to access device connection is initially established the access device will have allocated a unique Cell Radio Network Terminal Identity (C-RNTI) to the UA. With respect to the UA initiated communication, the UA has to first request to access the access device within a cell associated with the access device. To request an access, a UA initiates a random access (RA) process whereby the UA selects one of a plurality of predetermined code sequences called RA preambles randomly or via a predetermined rule and transmits the selected RA preamble on an asynchronous RA CHannel (RACH). When the access device receives the RA preamble, the access device transmits an RA response message including an RA preamble identifier (id or index) for the RA preamble, a timing advance value by which to adjust UpLink (UL) timing synchronization, grant information indicating UL resources allocated for transmitting subsequent messages, and a Temporary Cell Radio Network Terminal ID (Temporary C-RNTI) that is used as a temporary UA ID during the random access procedure. After receiving the RA response message, the UA checks the RA preamble id and if the checked RA preamble id is identical to that of the transmitted RA preamble, the UA transmits an uplink scheduling transmission to the access device. One exemplary type of uplink scheduling transmission includes a buffer status report (BSR) including the assigned C-RNTI in order to report amount of data in the UA's uplink buffer to be sent to the access device.
If a plurality of UAs transmit the same preamble to the access device at the same time, contention occurs in the RA procedure. When contention occurs, the access device resolves the contention and transmits a Contention Resolution (CR) message on the PDCCH to the C-RNTI of the UA that won the contention. Each UA having a C-RNTI can determine from the C-RNTI of the CR message whether it has won or lost the RA contention. If the C-RNTI of the CR message is not that of a UA, the UA has lost the contention and the UA re-starts the RA procedure. If the C-RNTI of the CR message matches the C-RNTI of a UA, the UA has won the contention and successfully completed the random access procedure.
With respect to the access device initiated communication, an access device can start an unscheduled communication by transmitting a downlink data arrival notification with a dedicated preamble on the PDCCH to the C-RNTI associated with a UA. When the UA associated with the C-RNTI receives a downlink data arrival notification, the UA recognizes that the access device has data to transmit to the UA and starts a random access process by generating and transmitting the dedicated preamble transmission (i.e., a preamble specifically assigned to the C-RNTI by the access device) back to the access device. The access device transmits an RA response when the dedicated preamble is received where the RA response includes, among other data, a timing advance value by which to adjust UpLink (UL) timing synchronization.
UAs are only allowed to transmit data at their allocated time intervals. If there is data to be transmitted, a UA temporarily stores the data in a UA data buffer and transmits the data using granted uplink allocations. From time to time, the UA reports to the access device an amount of data stored in the buffer in a BSR and requests an allocation of resource for transmitting the data. The access device allocates an uplink grant to the UA based at least in part on the amount of data reported by the BSR and communicates that grant to the UA. After the grant is received, the UA transmits data on the uplink shared channel in a manner consistent with the allocated grant.
In order to facilitate non contentious access request for a UA which maintains uplink time alignment or synchronization, the access device may periodically allocate an uplink resource to the UA during which the UA may transmit a scheduling request (SR) to the access device for requesting an uplink grant for transmitting the BSR or other uplink scheduling transmission to the access device when there is data in the UA's uplink data buffer. The SR uses on-off keying on the physical uplink control channel PUCCH. The access device is programmed to monitor for the SR during the uplink period and when no SR is received during the period, to recognize that the UA has no uplink data in the UA's buffer for transmission and the access device foregoes granting an uplink period for BSR delivery.
When an SR signal is detected during the allotted period, the access device assumes that the UA requires more uplink resources and grants an uplink resource for BSR delivery. After the uplink resource for BSR delivery is received at the UA, the UA transmits the BSR to the access device using the allocated resources. After the BSR is delivered, the access device identifies further uplink resources required to deliver the buffered data and may grant an additional uplink resource for transmission of the buffered data.
In the latest versions of E-UTRAN, an enhanced uplink channel is provided for supporting the scheduling mechanism and a hybrid automatic repeat request (HARQ) scheme. An example of HARQ is specified in 3GPP TS36.321, which is incorporated herein by reference. The HARQ scheme is used both in uplink and downlink in E-UTRAN. Take downlink transmission for example, for each protocol data unit (PDU) received, a positive acknowledgment (ACK) is transmitted on a Physical Uplink Control Channel (PUCCH) or a PUSCH from the UA to the access device after a cyclic redundancy check (CRC) performed by the UA indicates a successful decoding. If the CRC indicates a PDU is not received correctly, the UA transmits a negative acknowledgement (NACK) on the PUCCH or PUSCH in order to request a retransmission of the erroneously received PDU.
In the case of uplink transmissions the HARQ scheme is a bit more complicated and involves, in addition to positive and negative acknowledgements on a Physical HARQ Indicator Channel (PHICH), new transmission grants, retransmission grants, or no data on the PDCCH where UA behavior depends upon the data received via both the PDCCH and the PHICH channels.
In order to facilitate uplink transmission from a UA to an access device, an access device and a UA need to adjust the transmission timing so that the transmission reaches the access device at a certain time with an allowed margin for error regardless of the distance between the UA and the access point. To this end, the access device sends a Time Alignment (TA) command including a time advance value as a MAC Control Element when transmission timing adjustment is needed or periodically (see Sections 5.2 and 6.1.3.5 of 3GPP TS36.321) and the UA operates a Time Alignment (TA) timer. When a TA command is received, the UA applies the received TA value to restart the TA timer. If the TA timer expires, the UA recognizes that uplink time alignment or uplink synchronization is lost and releases control channel resources (e.g. PUCCH or SRS resources). Sounding reference signals (SRS) are transmitted from the UA to the access device to provide information on uplink channel quality. PUCCH and SRS are described in 3GPP TS 36.211, which is herein incorporated by reference. Uplink synchronization means that the UA maintains uplink time alignment.
Whenever data needs to be transmitted from a UA to an access device or vice versa, allocating resources quickly so that transmission can occur is extremely important and the industry is always searching for ways to eliminate unneeded steps in the allocation process. One circumstance that requires several process steps to resolve is where uplink synchronization is lost when the TA timer expires so that control channel resources are released and a new random access process must be performed.
Uplink synchronization can be lost either purposefully or via error. With respect to purposeful loss, an access device is programmed to facilitate optimal use of communication channels. One way to optimally use a channel is to cause a UA to release allocated resources (e.g. PUCCH and SRS resources) when the UA is not generating enough traffic to justify the resource allocation. To cause a UA to release resources, an access device may stop sending TA commands to the UA thereby causing the UA to release the uplink control resources configured for the UA without any explicit signaling to the UA.
With respect to loss of uplink synchronization via error, on a noisy channel, a TA command may not reach a UA but an access device may erroneously sense an ACK confirming delivery. Here, if the UA's TA timer expires prior to reception of a next TA command, the UA can lose uplink synchronization and release the control channel resources.
Subsequent to a UA releasing resources, the UA may need to transmit data to the access device. For instance, where synchronization is lost due to a NACK-ACK error at the access device while data resides in the UA's uplink buffer, the UA will need to immediately transmit data to the access device. As another instance, when the UA receives new uplink data the UA will need to transmit the data to the access device. Similarly, subsequent to the UA releasing resources, the access device may need to transmit data to the UA. Here, the access device transmits a new downlink data arrival notification to the UA and the UA responds by starting a random access procedure as described above.
Where an access device purposefully allows a TA timer to expire so that uplink synchronization is lost and a UA subsequently receives new uplink data for transmission to the access device or an access device subsequently receives new downlink data to be transmitted to the UA, as specified in 3GPP TS36.331 which is incorporated herein by reference, after a random access procedure is completed, an access device sends an RRC Connection Reconfiguration message to the UA and the UA replies with an RRC Connection Reconfiguration Complete message to reassign resources before data transfer starts.
Thus, it would be advantageous to have a system wherein the number of process steps to re-establish uplink synchronization after uplink synchronization is lost and the amount of data transmission needed to re-establish synchronization can be reduced.